On April 30, 1877, Charles
Cros submitted a sealed letter to the Academy of Sciences in Paris
describing how to reproduce sound from a recording of sound waves.

"The
letter stated in French, "Un index léger est solidaire du centre de
figure d'une membrane vibrante ; il se termine par une pointe [...]
qui repose sur une surface noircie à la flamme." The English translation
is one close to this: "A lightweight armature is fixed to the center
of the face of a vibrating membrane; it ends with a sharp point [...]
which rests on a lamp-blacked surface." This surface is integral with
a disc driven by a double movement of rotation and linear progression.
The system is reversible: when the tip follows the furrow the membrane
restores the original acoustic signal.

In his letter, after having
shown that his method consisted of detecting an oscillation of a membrane
and using the tracing to reproduce the oscillation with respect to
its duration and intensity, Cros added that a cylindrical form for
the receiving apparatus seemed to him to be the most practical, as
it allowed for the graphic inscription of the vibrations by means
of a very fine-threaded screw.

"As far as is known,
no one before him had thought of a practical way to reproduce sound
from a recording of airborne sound waves. He gave the Greek name 'Paleophone'
('voix du passé', tr. 'voice of the past') to his invention."

An article on the Paleophone
was published in "la semaine du Clergé" on October 10, 1877, written
by l'Abbé Leblanc. Cros proposed metal for both engraving tool attached
to the diaphragm and receiving material for durability" (1).

On December 3, 1877, the
sealed envelope was opened and the Cros letter was read in public.

The First Day cover below
celebrates the 100 year anniversary of the opening of that sealed
letter by the Academy of Sciences in Paris and is postmarked December
3, 1977.

Cros is therefore credited
"with anticipating, though barely, what Edison was to accomplish"
(2) which is why the First Day cover reads
100 years of the Phonographe (and not 100 years of the Paleophone
which was never built).

(1) Wikipedia

(2) Charles Cros, as described in “The Talking
Machine”, by Timothy C. Fabrizio and George F. Paul